Solvents have played an important roll in the development of the chemical industry. In particular, the importance of solvents in the fields of coating compositions, adhesives and printing inks has been ever increasing, with the recent remarkable development in the plastics industry. Coating compositions, adhesives, printing inks, etc. are generally used in the form of a solution in a solvent for assuring ease in handling on use, and for uniformly and intimately applying a high polymer base to a substrate. To this effect, it is very important for the solvent to have moderate volatility, while retaining sufficient dissolving power for the high polymer base and uniform coating properties, as well as ease in handling. In other words, the quality of coating compositions, adhesives, and printing inks largely depends on the choice of solvent.
Glycol ether type cellosolves, especially Cellosolve acetate (ethylene glycol monoethyl ether acetate), have been used for their excellent properties as solvents for cellulose resins, epoxy resins, acrylic resins, vinyl resins (e.g., vinyl acetate resins and vinyl chloride resins), alkyd resins, and polyester resins which are commonly used in the fields of coating compositions, adhesives and printing inks. In recent years, however, demand for safety of chemical substances has been increasing from the standpoint of environmental pollution. In this regard, use of Cellosolve acetate is strictly limited because of its toxicity, and the Japanese Industrial Safety and Health Law laid down criteria for controlling the working environment concentration thereof.
Intensive studies have therefore been directed to development of a solvent which can be a satisfactory substitute for Cellosolve acetate in terms of dissolving power, and yet gives rise to no safety problem. For example, ethyl lactate, propylene glycol monomethyl ether acetate, methoxypropanol, and ethyl .beta.-ethoxypropionate have been studied as promising alternatives. However, they are not always satisfactory in dissolving power, safety, smell, and ease in handling. Of these alternative solvents, ethyl lactate, which is permitted as a food additive, seems the most preferred from the standpoint of safety, but it is not deemed to have sufficient dissolving power for high polymers and various additives. While alkyl .beta.-alkoxypropionates, such as methyl .beta.-methoxypropionate and ethyl .beta.-ethoxypropionate, appear to be the most preferred from the viewpoint of dissolving power, they are still unsatisfactory in terms of dissolving power for high polymers or various additives, and in volatility after application. A mixture of methyl .beta.-methoxypropionate and ethyl .beta.-ethoxypropionate has been proposed as a solvent with improved physical properties, as disclosed in JP-A-3-284651 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, preparation of mixture involves a complicated operation, and is not suitable for industrial use. Thus, a practical solvent equal to Cellosolve acetate in performance has not yet been developed.
Besides the aforementioned applications, solvents are used in cleaning agents for removing oils, such as cutting oil, process oil, anti-corrosive oil, lubricating oil, grease and pitch, solder fluxes, inks, and liquid crystals.
Solvent-solubility of inks widely varies depending on their kind, for example, the kind of the base polymer or the hardening mechanism, such as ultraviolet-curing, heat-curing or hardening by evaporation. Therefore, an ink remover is must have a strong dissolving power to be applicable to any kind of ink.
In the production of liquid crystal displays, it is necessary to form a high-density electrode pattern on a glass substrate so as to make a fine display. However, it is not easy to keep dense lines of an electrode pattern insulated from each other. Existence of even a trace amount of a contaminant on the substrate causes display defects due to insufficient insulation or burnout due to continuous galvanic corrosion. This ultimately results in destruction of the display function. In particular, when a liquid crystal is injected into a liquid crystal cell, the liquid crystal adheres to unnecessary parts of the cell through capillary action. The adhered unnecessary liquid crystal, if left as such, will fail to provide a clear display image, and also contaminants in the air are taken up by dissolution, which tends to cause insufficient insulation. Therefore, the adhered unnecessary liquid crystal must be removed with a cell cleaner, but it is very difficult to completely remove the liquid crystal which has entered narrow gaps.
For these uses, solvents mainly comprising halogen type solvents, such as Freon 113 (1,1,2-trichloro-1,2,2-trifluoroethane), methyl chloroform (1,1,1-trichloroethane), and trichloroethylene, have been in wide use. In particular, Freon 113 has been used extensively because of its nonflammability, low toxicity, excellent stability, and dissolving power selective for various kinds of contaminants with no corrosion of metals, plastics or elastomers. However, because Freon 113 and methyl chloroform rise up to the stratosphere and destroy the ozonosphere, which eventually leads to induction of cancer of skin, their use has been severely restricted. Use of trichloroethylene is also now been restricted because it is suspected as being carcinogenic.
Accordingly, intensive studies have been conducted in order to secure a cleaning agent which will take the place of Freons, showing a cleaning action equal to the Freon 113, without entertaining a fear of destruction of the ozonosphere. For example, a number of substitutes for Freons have been proposed to date, including a composition mainly comprising 1,2-difluoroethane (see JP-A-1-132694), a mixture of 1,1-dichloro-2,2,2-trifluoroethane and dimethoxymethane (see JP-A-2-178396, corresponding to U.S. Pat. No. 5,068,051), and a composition mainly comprising hexafluorobenzene (see JP-A-3-167298). However, none of these solvent compositions offers a complete solution to the above-mentioned problems, i.e., they do not compare with Freon 113 in terms of performance. In addition, there is a movement to restrict use of these halogen type solvents.
On the other hand, cleaning agents for removal of fats and oils, i.e., degreasing agents, which are highly safe to humans and which do not cause environmental destruction have been proposed. For example, a composition mainly comprising a nonionic surface active agent and an alkyl lactate (see JP-A-4-68088), a composition mainly comprising a nonionic surface active agent and an adipic ester (see JP-A-4-59985), a composition mainly comprising a nonionic surface active agent and a polyalkylene glycol dialkyl ether (see JP-A-4-59984), a composition mainly comprising a nonionic surface active agent and N-methylpyrrolidone, etc. (see JP-A-4-68094), a composition mainly comprising a nonionic surface active agent and a glycerin acetate compound (see JP-A-4-68092), and a composition mainly comprising an alcohol and a fatty acid ester (see JP-A-4-68090) have been proposed. Although an alkyl lactate, N-methylpyrrolidone, etc. are highly safe solvents in terms of low toxicity, and do not cause environmental destruction or accumulate in the environment, they have insufficient dissolving power for fats and oils when used alone, as is obvious from the Comparative Examples given in the aforementioned patent publications. They essentially need a combined use of a detergent aid, such as a surface active agent, for application as a degreasing agent.
A solvent composition mainly comprising an alkyl lactate which is highly safe to humans and does not cause environmental destruction has been proposed as a cleaning agent for removing inks, i.e., an ink remover, as disclosed in JP-A-3-41170. While an alkyl lactate is a highly safe solvent in terms of low toxicity, and does not cause environmental destruction or accumulate in the environment, it is still unsatisfactory as an ink remover due to insufficient dissolving power for high polymer-based inks.
As discussed above, it has been desired in the art to develop a solvent system which substitutes for Cellosolve acetate, Freon 113, methyl chloroform, etc., while exhibiting a high dissolving power for high polymers, fats and oils, solder fluxes, liquid crystals, agricultural chemicals, cosmetics, and various compounding additives, without giving rise to safety problems.